1. Field of the Invention
The present invention relates to a negative electrode for lithium secondary battery and a lithium secondary battery using the same, and more particularly to a negative electrode for lithium secondary battery which has a large discharge capacity, a large output density and an excellent cycle characteristic. The lithium secondary battery is applied as a power source for electric automobile or motor bicycle, a memory backup power source, or a driving power source for portable equipment.
2. Description of Related Art
The negative electrode of the conventional lithium secondary battery is made of a lithium metal or an alloy such as Li-Al, Li-Pb or the like. However, such a secondary battery has the problems including the short-circuiting of opposite electrodes caused by the deposition of resin-like lithium, a short cycle life and a low energy density. Recently, an active study of the use of a carbon material for a negative electrode has been prosecuted in order to solve those problems. The references include, for example, JP-A-5-299073 and JP-A-2-121258. In the JP-A-5-299073, there is used as the material of a negative electrode a carbon compound material which is formed in such a manner that the surface of a high-crystalline carbon particle forming a core is coated with a film including a VIII-group metal element and the resultant structure is further coated with carbon. It is shown by the JP-A-5-299073 that the carbon material on the surface having the disturbed-layer structure assists the intercalation of lithium and the increased surface area results in a remarkable improvement in charge/discharge capacity and charge/discharge rate. On the other hand, in the JP-A-2-121258, there is used as the material of a negative electrode a mixture of a carbon material which has H/C (a Hydrogen/Carbon mole ratio) smaller than 0.15, a face-to-face dimension larger than 3.37 .ANG. and a crystal grain size or crystallite size Lc smaller than 150 .ANG. in a C-axis direction and a metal which is capable of being alloyed with lithium. It is shown by the JP-A-2-121258 that a charge/discharge cycle life is elongated and a charge/discharge characteristic in a large current is satisfactory. In either the lithium secondary battery disclosed by the JP-A-5-299073 or the lithium secondary battery disclosed by the JP-A-2-121258, however, the composition of the carbon material of the negative electrode is difficult. Also, the theoretical capacity of carbon is not achieved and hence an output density is insufficient. The use of such a lithium secondary battery in an electric automobile or motor bicycle is difficult because of the insufficiency of the energy density and the output density.